Add namespacet::follow_struct_or_union_tag to simplify follow_tag uses

Replacing namespacet::follow by namespacet::follow_tag produced several
instances that require expanding composite data types, without a need to
distinguish structs vs unions.

src/util/c_types.h

@@ -195,11 +195,11 @@ inline union_typet &to_union_type(typet &type)
 }
 
 /// A union tag type, i.e., \ref union_typet with an identifier
-class union_tag_typet : public tag_typet
+class union_tag_typet : public struct_or_union_tag_typet
 {
 public:
   explicit union_tag_typet(const irep_idt &identifier)
-    : tag_typet(ID_union_tag, identifier)
+    : struct_or_union_tag_typet(ID_union_tag, identifier)
   {
   }
 };

src/util/namespace.cpp

@@ -92,6 +92,15 @@ namespace_baset::follow_tag(const c_enum_tag_typet &src) const
   return to_c_enum_type(symbol.type);
 }
 
+const struct_union_typet &namespace_baset::follow_struct_or_union_tag(
+  const struct_or_union_tag_typet &src) const
+{
+  const symbolt &symbol = lookup(src.get_identifier());
+  CHECK_RETURN(symbol.is_type);
+  CHECK_RETURN(symbol.type.id() == ID_struct || symbol.type.id() == ID_union);
+  return to_struct_union_type(symbol.type);
+}
+
 /// Follow macros to their values in a given expression.
 /// \param expr: The expression to follow macros in.
 void namespace_baset::follow_macros(exprt &expr) const

src/util/namespace.h

@@ -21,9 +21,11 @@ class symbol_exprt;
 class tag_typet;
 class union_typet;
 class struct_typet;
+class struct_union_typet;
 class c_enum_typet;
 class union_tag_typet;
 class struct_tag_typet;
+class struct_or_union_tag_typet;
 class c_enum_tag_typet;
 class symbol_table_baset;
 
@@ -68,6 +70,10 @@ class namespace_baset
   const struct_typet &follow_tag(const struct_tag_typet &) const;
   const c_enum_typet &follow_tag(const c_enum_tag_typet &) const;
 
+  /// Resolve a `struct_tag_typet` or `union_tag_typet` to the complete version.
+  const struct_union_typet &
+  follow_struct_or_union_tag(const struct_or_union_tag_typet &) const;
+
   /// Returns the minimal integer n such that there is no symbol (in any of the
   /// symbol tables) whose name is of the form "An" where A is \p prefix.
   /// The intended use case is finding the next available symbol name for a

src/util/std_types.h

@@ -444,12 +444,56 @@ inline tag_typet &to_tag_type(typet &type)
   return static_cast<tag_typet &>(type);
 }
 
+/// A struct or union tag type. This is only to be used to create type-safe /
+/// APIs, no instances of this one can be created directly, use \ref
+/// struct_tag_typet or \ref union_tag_typet when creating objects.
+class struct_or_union_tag_typet : public tag_typet
+{
+protected:
+  struct_or_union_tag_typet(const irep_idt &id, const irep_idt &identifier)
+    : tag_typet(id, identifier)
+  {
+    PRECONDITION(id == ID_struct_tag || id == ID_union_tag);
+  }
+};
+
+/// Check whether a reference to a typet is a \ref struct_or_union_tag_typet.
+/// \param type: Source type.
+/// \return True if \p type is a \ref struct_or_union_tag_typet.
+template <>
+inline bool can_cast_type<struct_or_union_tag_typet>(const typet &type)
+{
+  return type.id() == ID_struct_tag || type.id() == ID_union_tag;
+}
+
+/// \brief Cast a typet to a \ref struct_or_union_tag_typet
+///
+/// This is an unchecked conversion. \a type must be known to be \ref
+/// struct_or_union_tag_typet. Will fail with a precondition violation if type
+/// doesn't match.
+///
+/// \param type: Source type.
+/// \return Object of type \ref struct_or_union_tag_typet
+inline const struct_or_union_tag_typet &
+to_struct_or_union_tag_type(const typet &type)
+{
+  PRECONDITION(can_cast_type<struct_or_union_tag_typet>(type));
+  return static_cast<const struct_or_union_tag_typet &>(type);
+}
+
+/// \copydoc to_struct_or_union_tag_type(const typet &)
+inline struct_or_union_tag_typet &to_struct_or_union_tag_type(typet &type)
+{
+  PRECONDITION(can_cast_type<struct_or_union_tag_typet>(type));
+  return static_cast<struct_or_union_tag_typet &>(type);
+}
+
 /// A struct tag type, i.e., \ref struct_typet with an identifier
-class struct_tag_typet:public tag_typet
+class struct_tag_typet : public struct_or_union_tag_typet
 {
 public:
-  explicit struct_tag_typet(const irep_idt &identifier):
-    tag_typet(ID_struct_tag, identifier)
+  explicit struct_tag_typet(const irep_idt &identifier)
+    : struct_or_union_tag_typet(ID_struct_tag, identifier)
   {
   }
 };